WASHINGTON -- A long-awaited federal rule designed to protect workers in the construction and mining industries has been tied up in red tape at the White House, leading scientists to worry that the rule has been left in limbo due to political concerns.

The rule put forth by the Labor Department would limit workers' exposure to crystalline silica, a dangerous breathable dust commonly found in sand, granite and other materials used in construction. For construction workers and sandblasters in particular, breathing the dust over the course of years has long been known to lead to silicosis, a respiratory disease linked to lung cancer and respiratory failure. Companies in the mining and construction fields are concerned the tighter regulations could lead to higher production costs.

The silica rule has been under review by the White House's Office of Management and Budget for more than 11 months. Such rules are supposed to be reviewed by the administration within 90 days and then undergo a public-comment period before being finalized. While this rule's been at the White House, the budget office has held nine closed-door meetings on the issue with interested parties, many of them trade groups such as the National Association of Home Builders, the American Chemistry Council and the National Industrial Sand Association.

Proponents of the rule have grown so worried about the holdup that more than 300 occupational health experts, public health advocates and labor officials signed a letter sent Wednesday to the White House urging President Barack Obama to release the rule for public comment. Laying out the scientists' "serious concern" with the rule's "extraordinary delay," the letter notes that an estimated 1.7 million American workers are exposed to silica and roughly 200 die from silicosis each year, according to statistics from the Centers for Disease Control.

"These closed door meetings with special interests are wholly inconsistent with your promise of openness and public participation," the letter states. Those signing included officials from the American Medical Association, various occupational health and safety boards, and the nonprofit Union of Concerned Scientists, which advocates for sound science in policymaking.

Several observers, some of who signed on to the letter, told HuffPost the White House may not want to release a rule that would annoy deep-pocketed interest groups just as Obama's 2012 presidential campaign gets under way. In an email, OMB spokeswoman Meg Reilly declined to comment on the status of the rule. "It's not uncommon for review periods to be extended for regulatory actions that require additional time for consideration of public comment and analysis by OMB and all the affected agencies," she wrote. "I don't know precisely when review will conclude."

"It affects a lot of industries, and they'll say this could be a very expensive rule," explained Celeste Monforton, a former Labor Department safety official now with the George Washington University School of Public Health. "That's a legitimate point. But more important is to have time for the public dialogue, and not have the debate through these back-door meetings."

The debate over the silica rule highlights some of the dissatisfaction on the left with what they see as the Obama administration's sluggishness on regulations. Although the president has been blasted by Republicans for his alleged regulatory zeal, the White House in reality has issued relatively few new rules, especially pertaining to labor and the workplace. In fact, many safety advocates feel that the administration bends too often to industry when it comes to putting workplace protections. According to Wednesday's letter from safety experts, the administration has proposed "no new significant ... worker safety and health rules" and "promised rules have been repeatedly delayed."

"The Obama administration has been bad on regulations in general," said Justin Feldman, workplace safety coordinator for the watchdog group Public Citizen, which has tracked the silica issue for years. "There are people within the administration who held genuinely anti-regulatory views. I think [the silica rule] is up in the air."

Feldman said he's concerned that the administration won't release the rule before the election -- and that Obama might possibly lose. "That's what we're worried about," he said. "Romney or Gingrich would be very unlikely to publish this rule. The construction industry does not like it."

The silica rule isn't the first politically sensitive safety regulation to undergo a lengthy review at the Obama White House. Controversial child labor rules that would limit the work activities children could perform on farms and in grain facilities were tied up at the White House for nine months, until finally being made available for public comment in August. Although workers' and children's advocates said the rules are decades overdue, many farmers oppose the federal encroachment. If the White House had political concerns, they were well-founded. One GOP candidate for Congress in Arkansas has gone so far as to make repealing the child labor rules a central part of his platform.

According to Feldman, tighter silica rules have been under consideration by the Labor Department for at least eight years. Although few know the exact language of the proposed rule because it hasn't been released, experts said it's probably modeled on a California state law already on the books. That law requires that construction companies mitigate silica dust through better ventilation and so-called wet cutting -- that is, wetting down the brick and concrete before cutting it to reduce the dust clouds commonly seen on construction sites.

The dangers of intense or prolonged silica exposure have been known for decades. In the 1930s, hundreds of workers in West Virginia were believed to have died due to exposure to silica dust while building the Hawks Nest Tunnel, making silicosis a national issue at the time. Experts now liken it to the better-known black lung disease, or coal worker's pneumoconiosis

"It's a silent disease in the sense that a lot of workers are exposed to silica dust and just feel it as an annoyance," said Robert Harrison, M.D., a clinical professor at the University of California-San Francisco who has treated workers with silicosis. "It irritates the throat, but they don't realize it has long-term effects. ... It can cause disability, difficulty breathing, chest tightness, cough -- it has a real impact on someone's everyday life. And it's also a carcinogen."

Among those now suffering from silicosis is Leonard Serafin, a California resident who worked for a railroad for 32 years. It was part of Serafin's job as a trackman to lay out the ballast, or crushed rock and gravel, in which the tracks were placed. He often spent hours a day breathing in dust from the ballast cars, and he said he now suffers from chronic obstructive pulmonary disease and a host of other lung-related ailments.

At the urging of an occupational health expert, Serafin drafted a letter explaining his condition and his support for a new silica rule. The Serafin family shared the letter with HuffPost. In it, Serafin writes that his chronic cough has affected "all aspects of my life," making it difficult even to do household chores, let alone anything that requires exertion.

"I never dreamed I would have to spend my retirement years in this debilitating manner," Serafin writes. "I find it difficult to attend social events such as concerts and plays with my family because of my chronic cough. Even coughing while standing at a cash register line at a retail store causes people to distance themselves from me. ... When I exert myself, my daily coughing becomes a spastic type of cough, which leaves me exhausted, breathless with chest pain."

In closing, Serafin urges the White House to conduct its review and draft a regulation as soon as possible. "You have the power to prevent thousands of new cases of silicosis," he writes. "In good conscience, how can you put a price tag before the safety of U.S. workers?"